


for yonder breaks a new and glorious morn

by blackorchids



Category: Life with Derek
Genre: Christmas, Christmas Eve, Christmas Fluff, Established Relationship, F/M, Family Dynamics, Family Feels, Fluff and Smut, Friends With Benefits, Missing Scene, Mutual Pining, Porn with Feelings, Post-Episode: s03e16 A Very Derekus Christmas, Warm and Fuzzy Feelings, blended families - Freeform
Language: English
Status: Completed
Published: 2017-12-25
Updated: 2017-12-25
Packaged: 2019-02-20 14:44:01
Rating: Explicit
Warnings: Underage
Chapters: 1
Words: 2,954
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/13148853
Author URL: https://archiveofourown.org/users/blackorchids/pseuds/blackorchids
Summary: It's the season of giving, apparently, so Derek is maybe willing to have a talk about f-f-feelings. Only after Casey takes her top off, though.





	for yonder breaks a new and glorious morn

**Author's Note:**

> title from the song _O, Holy Night_ , and I prefer the Mercy Me version, which you should all give a listen to, if you've never heard it!
> 
> the underage warning is because they're both 16-17 in this, don't worry.  
> this is shameless and indulgent and I hope you guys like it.

Derek sort of feels bad about stomping up the stairs after completely blowing off his dad, Nora, and Aunt Madge, but it’s completely overtaken by the fact that he and Ralph hadn’t been able to even get out of Ontario before Ralphie had been all but forced into a ditch, trying to avoid a collision with a pinwheeling Sedan.

All of that flies out of his head when he slams his bedroom door shut and slumps against it, groaning for a long, drawn-out moment before he’s interrupted by _someone’s_ prudeish cough for attention.

Lounging across his bed, which has since been made in the time between when he’d left that morning and right that instant, is Casey, paused in flipping through one of his car magazines, looking more than a little annoyed to see him.

The fact that the entire view, bunny slippers and disdain and all, has popped into his head during more than one of his solo Derek evenings does not keep him from scoffing, full of that false bravado that gets her _really_ irritated. “I’m not in the mood, sweetcheeks,” he says, tilting his head to the side and shooting over a simpering apologetic look that has her mouth—god, stop staring at her mouth, Venturi—twisting up into a familiar sneer.

“ _Your_ aunt Madge has taken up residence in _my_ room, and _you’re_ supposed to be out of the country by now and unable to get back through customs!” She hisses bitchily, tossing aside his magazine like she wouldn’t have a fit if he treated one of _her_ books with so much carelessness. She pushes up onto her hands and knees—good thing he’s an expert at not gawking—before rocking back on her haunches, tossing her hair and crossing her arms under her chest.

She knows what she’s doing too, which makes it easier to kick her out. They haven’t fooled around since before Max, but he’s definitely _not_ going to be anything even remotely on the same page as a _rebound_. He doesn’t have much, but what dignity he does have he’s not willing to part with it so fast.

Derek shuts the door too quick, gets her on her ass and can hear her split his name into two parts as she stomps across the landing to poor Lizzie’s room.

He texts Sam, complaining about how he and Ralph hadn’t made it past the border because of the weather. Sam’s a filthy traitor, though, because he worries idly that, if the weather was too bad to drive in, Casey and Lizzie’s flight might not do so well.

 **[To: sammy boy | 18:29]** idk ne thing abt tht  
**[To: sammy boy | 18:29]** ull hafta ask her royal prissiness

 **[From: sammy boy | 18:30]** so ur saying she’s got signal

 **[To: sammy boy | 18:30]** im not saying ne thing

Downstairs he can hear too-loud Christmas music playing on their crappy old boombox in the kitchen. Clearly Nora or his dad is trying to get the rest of the Venturi-McDonald clan in the holiday spirit, but he can already imagine how his dad’s _presents after the holiday_ idea is going to go down with the kiddies, and if Casey’s in such a bad mood, the rest of them will surely follow through, whether they’re aware of it or not.

The music stops suddenly and Aunt Madge’s voice sounds sharper than he’s heard her sound in years, so Derek lunges to the door, opening it as quietly as possible so he can try and listen in. She’s berating them for being ungrateful little brats, he bets.

“Uh,” Casey’s saying, sounding incredibly dubious. “Aunt Madge, I’m the last person in the _world_ who could convince Derek to do something he doesn’t want to do.”

Derek doesn’t laugh, because it’s honestly not funny that she’s still so completely clueless, but he does dive for his bed and get himself situated, because Casey’s never met an adult she could say no to.

Sure enough, she comes barging into his room, sounding annoyed and impatient already and the conversation hasn’t even started. Derek considers himself an expert in agitating Casey, sure, but he can’t help but feel this intense irritation is a little unfair.

“I can’t hear you,” he says serenely, not looking over at her. “I’m on the beach.” If his mental beach has Casey in a stringy bikini and for-sure over Max, no one has to know but him, and he’s ashamed enough for everyone.

She goes for the party-animal-Derek argument, but her voice is weak and altogether unconvincing, because he bets she’s still not very much in the holiday spirit either, even if she’s going to look down at him from her high horse and pretend like she’s ready for blended-family festivities.

He’s just annoyed enough that he’s actually mean to her after she calls him selfish, and she flinches back a little, looking away from him and frowning contemplatively as she—god almighty— _actually takes his words to heart_.

Hearing her tell everyone goodnight is about when Derek decides he’s going to save Christmas, even if he’s going to keep calling it Derekus out loud. Fortunately, it’s not Casey coming to argue with him some more, because if she cries he’s in just the right sort of mood to apologize, and that’s not really what they do.

His aunt Madge comes into his room, looking around curiously before laying on the flattery really quite thick. He pretends he doesn’t have a double meaning hidden in his words when he says it just might not be meant to be, and manages to keep a straight face when she agrees without hesitation.

He tells her that her guilt trip isn’t working, but he’s always been a sucker for the women in his family, so he asks for an out and she’s more than happy to give it.

Casey and Lizzie are a pair of sad sacks of girl in Lizzie’s bedroom, and while Lizzie is obviously disappointed she’s not seeing her father, Casey’s voice is a little scratchy, like she’s been crying and trying not to let anyone see, which is what she does when she’s _actually_ upset instead of just being dramatic. His stomach twists up unpleasantly, the sour taste of guilt lingering at the back of his mouth, but he snaps briskly and tells her to get over herself because that’s just how they are.

Surprise at his sudden reinterest in the holiday startles her out of her slump, but the expression on her face after Derek tells her aunt Madge _just asked_ for help doesn’t help with the guilt factor. She really doesn’t know anything.

He pushes that aside though, and puts them all through their paces, assigning roles in his no-nonsense director’s voice and pretending he doesn’t feel her impressed gaze on the side of his face when he patiently but quickly coaches Ed and Lizzie through the beginning of the show. It’s his turn to watch her when she helps Marti dress up in some of her clothes, taking accessorizing as seriously as Marti does, offering suggestions and listening intently to Marti’s opinions.

“There are five kids now, and they actually get along pretty well,” he says, finishing off the tiny show in his best announcer’s voice and Casey smoothly rises up from her knees—mind out of the gutter, Venturi—to politely remind everyone that _no_ , Derek and Casey actually don’t get along, sorry.

“Except for Casey and Derek,” he agrees in a more normal voice, trying not to look over at her and trying not to look like he’s thought entirely non-sisterly thoughts about her. Luckily, the adults are clapping and cooing, commenting over Edwin’s wig and Marti’s necklaces, and all he has to do is bow a few times and ham it up. 

His aunt Madge thanks him entirely too earnestly for his comfort zone, so he pushes a hat on over her hair and invents a dance at the top of his head. Casey’s right behind him, on the same wavelength like always, easily following his dance routine.

After dinner and putting the little sibs to bed, she comes into his room, quiet and shy and smaller than usual in her ridiculous holiday-themed pajama outfit, her hair pulled back into a braid.

Derek can tell she’s got something to say, but he can’t guess at what it might be and that makes him edgy, so when he gestures for her to sit down, he obstinately doesn’t move away from the center of the bed, daring her with his gaze to take the safety of the desk chair.

She sits on the corner of his bed, looks at the palms of her hands, heaves a huge breath.

“I’m not really upset about Max,” she says, and her inflection is off, because it sounds like she’s telling him he’s wrong, when he _knows_ she cried over the boyfriend comment earlier that evening.

“I’m used to it, by now,” he tells her, shrugging carelessly, making to go back to his magazine. She slaps it down, palm on his chest, separated by forty pages of glossy car photos and a single thin cotton baseball shirt.

“The boyfriend comment didn’t upset me _about Max_ ,” she says again, sounding frustrated. “You—he—”

“I really would rather not talk some more about the fascinating saga that is Casey McDonald and her fantastically disappointing boyfriends, if you don’t mind,” Derek says. Part of him wants to call her _sis_ , because that always gets her to back off, and the other part of him is aware that it’s a few minutes until Christmas and even at his worst he would rather try and avoid making Casey cry on Christmas.

“You wouldn’t be just a rebound,” is what she settles on, nodding once like she’s come to an agreement with herself. She looks a little defiant, like she’s challenging him to deny her claim, but he’s too busy _having a heart attack_ , because she’s actually, verbally, alluding to the pair of them in more than a step-sib context. She looks away again, returning her blue-eyed gaze back to the palms of her hands. “You have never been just a rebound,” she says, quieter even than her last statement, clearly having lost most of her bravery.

Except this is obviously the bravest thing she’s ever done, and he’s at once proud of her and indignant with her, because now it’s on him. If he says the wrong thing, they won’t have happened _because of him_ , and if he manages to fumble through saying the right thing, they’ll have to eventually come clean to the family _because of him_.

Casey has handed him complete control over their bizarre situation, and he has never wanted to be in charge less.

She looks so nervous though, bracing for a rejection and ready to escape his bed and his room as soon as he tells her to leave, and he guesses that it would put an indefinite moratorium on the pair of them ever being anything more than housemates, because they’ll never be able to be siblings.

He’s taking too long, thinking too much—how ironic, honestly—because she starts to get up, mumbling a half-hearted _I just wanted you to know_ , but he manages to get with the program enough to lean forward and wrap his fingers around her wrist, tugging her towards him that she ends up in an ungainly heap atop him.

“This was completely accidental,” he blurts out before she gets too scandalized and she snorts, shifting and adjusting with really too much rocking of her hips against his own. She settles into a familiar spot atop him, one leg on either side of his, sitting backwards on her haunches once more, watching him curiously.

And suddenly, before he says anything and they get into an argument and roll around on his bed somewhere between wrestling and grinding for too long a while, he absolutely has to kiss her. It’s been months and months, and he knows him wanting to follow Sally to the fucking British Columbia had hurt her way more than her fooling around with Max had hurt him.

 _I’m sorry_ , he says to her mouth, quiet enough that he can claim denial if she’s out of practice enough to mention it, but she knows better than that, by now, and just leans forward, getting a hand in his hair and kissing him back, forgiveness and hopefulness in every gentle swipe of her thumb against his jawline.

He tilts her head to the side with a gentle-but-firm grip on her braid, kisses down the slender column of her neck, the scent of her lotion settling around him in a haze. He thinks about one day being able to bite into the smooth skin of her collarbones, leave a bruise there for Emily to giggle over and people to notice, wonders if that should technically be a part of his no-PDA rule and why it’s not.

Casey lets him work the laughably large red buttons of her ridiculous pajama top, smirks a little when he groans upon finding she’s wearing nothing underneath. Her smirk falters when he gets a hand on one of her breasts, thumbing over a nipple carefully at the same time as he scrapes his teeth down the line from her ear to her shoulder.

She arches her back and presses down against his hard-on as bold and unafraid as always, scraping her nails down one of his arms as she gasps and grinds against him, driving him crazy. His head is full of her, his senses overtaken, and she’s whispering his name like a prayer, splitting it apart much the same way as she does when she’s yelling at him—ask him why he takes so much pleasure in annoying the shit out of her.

Casey is at once vulnerable and bossy like this, knows what she wants and isn’t too shy to ask for it but comprised of slim taut muscle and delicate feminine curves and oddly small when she’s out of her bulky pajamas. Derek knows he’s scrappy, but Casey likes to grip his shoulders, and he knows he’s slowly getting more broad, and, more importantly, he knows what she likes, which is to be on top until she’s relaxed and clingy with the first orgasm.

She clearly thinks he’s taking too long with her breasts, tries to guide his hands further down her body, skating his palms over her curved side and her toned belly and arching a brow when he doesn’t immediately sink his fingers beneath the elastic waistband of her stupid, stupid pajamas.

Sue him, he wants to take his time, because Casey, half-undressed and skin bereft of makeup and bunny slippers still on her feet because her toes are always cold—well, he likes all of the Caseys, finds her keener abilities as endearing as he finds them frustrating, cannot help the bitter pride when she’s dating someone else and he looks them in the eye and _doesn’t_ mention he’s seen her naked, has a fondness for her skirts and any time she wears those skin-tight dancing outfits, but Casey like this, relaxed and sleepy and completely uncovered after sneaking into his bedroom?

He’d never admit it, but this might be his favorite Casey of them all.

After he finally gets a hand in her pants, it’s short work until she’s coming, a blissed out expression on her face and his name falling from her mouth on one long, drawn-out exhalation, and she lets him turn them over so she’s cradled between his pillows, her hair escaping from her braid and fanning out around her head.

Derek can’t stop kissing her, barely even cares that he’s still hard and that she’s not going to let them have sex because the family is home, can’t stop hearing her voice playing in his head, over and over _you have never been just a rebound_ , cycling around his brain like it’s on a banner, being tugged by a tiny brain-plane.

Barely cares, because he’s still a teenage boy, and he still loses the grace of the kiss when she gets a hand on him, expert level at getting him off because she knows just as much about what he likes as he does about her, and she wraps her legs around his waist even though it can’t be comfortable with her arm at that angle, and she guides his face into the crook of her neck, fingers tugging on the too-long hair at the nape of his neck.

Derek comes too quickly, and, whenever they’ve done this in the past, he’s always been overtaken by a frantic urge to keep going, to get her off as many times as she’d let him, so he’s sure she wouldn’t be able to just write him off as some casual teenaged fumbling, but she keeps her hand in his hair and wipes her other one off on his sheets—thanks, Case—and keeps herself wrapped up around him like a particularly sticky octopus, like she’s comfortable with him lying atop her and panting slightly into her neck.

After a few minutes where he lets his heart rate return to a more steady pace, he pulls himself up just a little so he can kiss her some more, pressing quick intimate kisses to her mouth, the pair of them trading back and forth, simply revelling in sharing the stolen moment with one another.

“Merry Christmas, D,” she whispers when they break for air, and he presses his lips against hers for a longer kiss, hand curling over her side possessively, not that he’d ever admit it.

“Merry Christmas, Case.”

**Author's Note:**

> happy holidays loveys!! come leave the gift of a prompt on my [tumblr](http://www.rosalinesbenvolio.tumblr.com)!! *wink wink*


End file.
